Determining Correct Scope Base Angle

By Robert H. Thompson

During a recent rifle project I wanted to know exactly how much angle correction I needed in my scope base to center the elevation settings that I would be using, within the scopeís adjustment range. This was an economy long range rifle build and I was going to use shims under the rear scope base to get the angle correction needed. I was using Burris 30mm Signature Zee rings, which allow some adjustment with their offset inserts, but I had already used some of that feature to center the windage adjustment, and the 30mm inserts didnít offer as much correction as I wanted.

This picture shows how the Burris Signature Zee Rings with their synthetic inserts allow for considerable angle changes. I like these rings for several reasons, they donít mar the scope, they donít induce any forces into the scope from miss aligned bases, and they allow for angle correction using the offset inserts.

To work out how much correction I needed for this rifle, I created a simple drawing which helps to graphically illustrate the relationship of elevation settings with and without an angled scope base. This drawing can be easily duplicated for any rifle/scope ballistics combo.

In the above example Iím using my SWFA Super Sniper scope on a 308 rifle shooting a 168 grain bullet. My Super Sniper has 144 MOA of elevation adjustment, represented in the drawing by the vertical line with horizontal lines laid out similar to the scopeís turret, minus a lot of the hash marks for simplicity. The numbers represent revolutions of the vertical turret with 15 MOA per turn.

The three variables in this illustration are the scopeís range of vertical adjustment, the minimum and maximum shooting distances and the ballistics of the round being used.

After zeroing at 200 yards with standard bases, the elevation adjustment was at 75 MOA from the lowest setting, close to center of the elevation adjustment. Zeroed at 1000 yards (from the ballistic charts) would equal roughly 50 MOA of come up or 125 MOA total. Both of these settings are noted on the drawing. I have also shown where the 200 and 1000 yards zero settings would be if they were centered in the elevation adjustment range. Simple math gives us the difference between the two settings, which in this case is 30 MOA.